Tubular electric resistance heating element



May 27, 1941- c. BQBACKER TUBULAR ELECTRIC RESISTANCE HEATING ELEMENT Filed Jan. 30, 1959 Fig. 4.

Fig. 3.

Patented May 27, 194i TUBULAR ELECTRIC RESISTANCE HEATING ELEMENT Christian Bergh Backer, Brornley, England Application January 30, 1939, Serial No. 253,526 In Great Britain February 1, 1938 1 Claim.

This invention relates to tubular electric resistance heating elements of the kind in which a coiled resistance wire, which is surrounded by insulating material, is enclosed in a metallic tube or sheath.

For many purposes tubular electric heating elements must withstand temperatures which may amount to from BOO-850 C. The tube or sheath in which the heating element and the insulating material are enclosed must then be capable of resisting temperatures up to 900 C. and must therefore be made of a heat resisting material, for example a nickel-chromium alloy or stainless iron or steel of suitable quality. To draw tubing from such heat resisting alloys is extremely diflicult and the cost of a seamless drawn tube, or even of a welded tube which must be drawn after welding, may amount per pound weight to 8-10 times the cost of the alloy in sheet form. The tubing therefore is too expensive in most cases to be used in the manufacture of heating elements. On the other hand, the cost of such heat resisting alloys in the form of sheet is not prohibitive.

I have now devised a method whereby tubing I can be made from such heat resistant alloys at a cost which is suiliciently low to allow the tubing, which is very suitable for use in the manufacture of tubular electric resistance heating elements,

tobeusedforthispurpose. Thetubingismade from a sheet or strip of heat resisting alloy by bending the sheet or strip into the form of a tube which may be of circular or other cross section, in such a way that the longitudinal edges of the sheet or strip lie in contact with one another and form a protruding rib on the outside of the tube. The protruding edgeswhich form the rib are then welded together, preferably by an electric seam welder, whereby the tube becomes watertight.

The invention thereforeconsists in an electric resistance heating element which comprises a coiled resistance wire which is surounded by or embedded in insulating material and enclosed in a tube made in the manner described from a sheet or strip of heat resisting alloy, for example a nickel-chromium alloy or a suitable stainless iron or steel alloy.

For certain purposes the protruding rib may not be desirable but in the majority of cases the presence of the rib is not disadvantageous andit may even be an advantage. In any case the rib helps to stiffen the tube and prevent it from bending, while, for many types of heating elements such as tubular boiling plates, grill element's, room heater elements and so forth the rib affords a convenient means of fastening the element, for example by spot welding, or otherwise, to a supporting frame or structure.

The presence of the .rib does indeed increase I the difliculty of bending the tube, which will usually be necessary, nevertheless it has been found that, if proper bending tools are employed, the tubing may be bent to any required shape andeven very sharp bends may be made. The rib also may be positioned in any desired plane with respect to the plane of thebend, that is to say, the tube may be bent with the rib either on the outside or on the inside of the bend or the rib may be perpendicular to the plane of the bend. The latter type of bend is the easiest to make because the rib is only stretched to a very small extent. When the rib is on the outside of the bend it has to be stretched considerably, especially if the bend is sharp, while when the rib is on the inside it has to be compressed. It has been found that the tubing may be bent to a radius which is slightly more than half the outside diameter of the tubing which is quite sui'ilcient for the manufacture of resistance heating elements of the usual designs.

In order that the invention may be readily understood and carried into effect it will now be described by way of example with reference to the accompanying drawing, in which Figure l is an end view showing how the tubing is formed by bending a strip or sheet of heat resisting metal.

Figures 2, 3 and 4 are views showing a bent tube with the protruding rib disposed in different planes relatively to the plane of the bend, and

Figures 5-, 6, 7 and 8 are cross sectional views of four different forms of tubular electric resistance heating elements constructed in accordance with the invention.

Referring to Figure 11 of the drawing, in order to form the tube I the longitudinal edges 2 and 3 of a strip of heat resisting metal or alloy are first bent at an angle of approximately to the plane of the strip. The strip is then bent into tubular form so that the longitudinal edges 2 and I lie in contact with one another along the line indicated by the numeral 4, after which they are welded together. The tube which has been manufactured in the manner described with reference to Figure 1, can be bent with the plane of the rib disposed in various positions relatively to that of the bend, as illustrated in Figures 2, 3 and 4.

The bending should of course be undertaken after the resistance wire and insulating material have been inserted in the tube.

Figures 5, 6, 7 and'8 illustrate diflerent forms of heating elements in accordance with the invention. In these figures, the numeral I designates the coiled resistance wire, i the insulating ma-- terial and 1 the tube which is manuiactured in the manner described with reference to Figure 1. In the manufacture of the electric heating elements shown in these figures the heating coil and the insulation are first enclosed in a circular tube of the kind shown in Figure 1, after which the tube is deformed in a suitable press to the shape shown in any of the Figures 5, 6, 7 and 8. During the shaping process, the insulating material in the interior of. the tube is simultaneously compacted. The rib 8 may be used for fastening the element to any support, for example by spot welding and in addition it assists in stifiening the tube.

Flor producing the insulation between the remam sistance coil and .the tubular casing and compacting it the methods described in the specification of my prior British Patents Nos. 462,263 and; 482,280 may advantageously be employed.

I claim:

The method of making an electric heating element which consists in bending two opposite edges 01' a sheet of metal to form a flange portion on each edge, bending the portion of the sheet between said flanges into tubular form to provide an outwardly projecting rib by contacting said flanges throughout their length, securing the flanges together to produce stillness of the finished tube, disposing a heating conductor within the tubular portion, packing insulating material around the conductor and bending the tubular portion to form a heating element.

CHRISTIAN BERGH BACKER. 

